1. Field of the Invention
This invention relates to controlling forebody vortex asymmetry in high performance aircraft and other flight vehicles and more specifically to controlling such asymmetry through the introduction of plasma discharges on the aircraft forebody.
2. Description of the Related Art
Aircraft designed for high-speed flight and combat generally have pointed forebodies and swept wings. During maneuvers at subsonic speeds, examples of which include the landing approach and combat, such aircraft fly at high angles of attack with respect to the flight path. When a pointed forebody is placed at an angle of attack generally exceeding 10 degrees in a fluid flow, a pair of vortices forms on its leeside. Each vortex induces a region of low pressure on the adjacent surface, the pressure coefficient being related to the strength of the vortex and its proximity to the surface. At high angles of attack, these vortices develop in an asymmetric manner, so that a net side force is induced on the forebody. The product of this side force and the distance to the center of gravity of the aircraft is a yawing moment. An additional and separate effect is the interaction of the asymmetric vortices with the flow over the wings of the aircraft that causes asymmetry in the lift between the wings of the aircraft. This asymmetry in lift produces a rolling moment on the aircraft. A third effect, related to the first, occurs when the aircraft nose is yawed at high angle of attack with respect to the flight direction, the side force on the nose also produces a rolling moment.
As shown in FIG. 1, if an aircraft 10 is in level flight, the angle that a flow vector 12 (oncoming flow of air) makes with the aircraft""s reference line 14 (the longitudinal axis between its nose and tail) is nearly zero. If the pilot pulls the stick back, the aircraft will pitch up and will reach an angle of attack xcex1 (the angle between the reference line 14 and the incoming flow vector 12) as shown in FIG. 2. If the aircraft 10 is maneuvering at a small angle of attack, the flow 16 around the aircraft is such that the boundary layer does not separate and therefore no vortices form as shown in FIG. 3. When the aircraft 10 maneuvers at a large angle of attack such as during a dogfight or when landing, the boundary layer 18 (a thin layer in which the viscosity of the air retards the airflow near the airplane surface) of flow 16 grows as the flow moves downstream and eventually breaks away or xe2x80x9cseparatesxe2x80x9d at separation points 20, forming eddies and vortices 22 in the flow that spin around themselves like miniature tornadoes as shown in FIG. 4. Separation and thus the formation of vortices occurs when the angle of attack exceeds the forebody half angle xcex4 in FIG. 2. Generally, when the angle of attack exceeds twice the half-angle, the vortices are asymmetric in both position and strength. These vortices not only extract energy from the flow and produce wind resistance termed xe2x80x9cdragxe2x80x9d but they vibrate the aircraft in a way that can weaken it and make it hard to control. Separation can be particularly acute near the nose or xe2x80x9cforebodyxe2x80x9d of the aircraft.
Most fighters have sharp slender noses to reduce drag. However, this makes the separation occur more readily. The biggest problem is that even if the aircraft 10 is symmetrically aligned to the relative wind on the pilot""s left and right side (port and starboard) termed, zero yaw, the flow 16 and vortices 22 can separate asymmetrically as shown in FIG. 5. Asymmetric vortex configurations can yaw the aircraft so much that its tail will face the flow rather than its nose. This condition is termed xe2x80x9cadverse yawxe2x80x9d or xe2x80x9cyaw departurexe2x80x9d. It may cause the aircraft to go into an uncontrollable spin in an upside down or xe2x80x9cinvertedxe2x80x9d position. In turn, this can cause the engines to stop (xe2x80x9cstallxe2x80x9d), leading to a crash. Generally this occurs so rapidly and with such force the pilot will not have enough control force or xe2x80x9cauthorityxe2x80x9d to quickly restore the aircraft to its equilibrium flight state.
A number of different techniques have been developed for controlling and reducing vortex asymmetry. These include passive strakes (see U.S. Pat. No. 4,225,102), deployable strakes (see U.S. Pat. Nos. 4,015,800; 4,786,009; 4,917,333; 5,207,397; 5,449,131), non-circular nose cross-sections (see U.S. Pat. No. 4,176,813), tiltable/rotatable noses (see U.S. Pat. Nos. 4,399,962; 4,579,298; 4,756,492; 4,793,571; 4,925,139; 5,050,819; 5,139,215; 5,139,216; 5,794,887) thruster jets (see U.S. Pat. No. 5,273,237) and other techniques (U.S. Pat. Nos. 5,201,829; 5,326,050). These patented techniques are effective to correct and reduce vortex asymmetry with varying degrees of success. However, even the best systems can occupy considerable space in the forebody of the flight vehicle, add considerable weight to the vehicle, consume large amounts of power, and have reliability issues associated with their mechanical complexity and compromise mission performance as well as other stability and control characteristics. High-performance fighters requiring high agility and other flight vehicles urgently need control systems that enable rapid and precise control to manage and overcome vortex asymmetry, with much less power, obtrusion, weight, performance degradation and mechanical complexity than current techniques. If these techniques can be developed, military vehicles can develop incredible agility and mission survivability. Commercial and general aviation vehicles also can be safer with these techniques since they frequently encounter high angle of attack environments where stallspin and yaw departure have caused frightening accidents.
Most aircraft have a vertical tail and controllable rudder. The tail itself provides a stabilizing influence to offset the vortex asymmetries. The rudder is used to coordinate turns as well as create lateral forces and rolling moments to control the yaw and roll of the aircraft. Under certain maneuvering conditions, even full deflection of the rudder will not provide adequate lateral control. Some aircraft use vectored thrust to supplement the rudder under these conditions. This sacrifices speed, energy and requires additional structure to withstand heat loads and high temperatures associated with the vectored thrust.
The present invention provides a system and method for rapidly and precisely controlling vortex symmetry or asymmery on aircraft forebodies to avoid yaw departure or provide supplemental lateral control beyond that available from the vertical tail surfaces with much less power, obtrusion, weight and mechanical complexity than current techniques.
This is accomplished with a plasma discharge to manipulate the boundary layer and the angular locations of its separation points in cross flow planes to control the symmetry or asymmetry of the vortex pattern. A closed-loop feedback control system that incorporates these principles includes three primary components; pressure sensors, a PID controller, and plasma discharge elements. Pressure sensors distributed around the forebody that include port and starboard locations provide information about the lateral symmetry of the pressures and vortices. The pressure data is fed to the PID controller to calculate and drive voltage inputs to the plasma discharge elements, which provide the volumetric heating of the boundary layer on a time scale necessary to adapt to changing flight conditions and control the symmetry or asymmetry of the pressures and vortices. In the case of yaw departure avoidance, the PID controller controls the plasma to adjust the separation points to angular locations around the forebody that provide a robustly stable symmetric vortex pattern on a time scale that the asymmetries develop. Stability may be further enhanced by using a boundary layer tripping plasma spark discharge that insures that both port and starboard sides are turbulent. In the case of lateral control, the PID controller controls the plasma to adjust the separation points to angular locations around the forebody that provide an asymmetric vortex pattern that produces the desired supplementary lateral force and rolling moment.